Rolling cone bits may have teeth machined from the steel bodies of the cones. Rolling cone bits may also have tungsten carbide inserts press-fit into mating holes in the cones. Hardfacing has been employed on the gage surfaces of both types of rolling cone bits, as well as on portions of steel bit bodies for many years to resist abrasive wear. Hardfacing is also applied to the machined teeth. However, hardfacing is not applied to tungsten carbide inserts.
The hardfacing typically comprises granules of tungsten carbide located within a steel alloy binder. One method of applying the hardfacing to rolling cone bits has been to use an oxy-acetylene torch to melt a hardfacing tube or rod onto the steel. The hardfacing rod is typically a steel tube containing a filler comprising tungsten carbide granules. The temperature to melt the tube and bond the hardfacing to the steel of the bit in a prior art method for rolling cone bits may be in excess of 1500° C.
Another type of bit, often called a diamond bit, has a cast metal-matrix body and polycrystalline diamond cutting elements attached to the body, rather than rolling cones. The metal-matrix material typically comprises tungsten carbide powder and a binder of a metal, such as copper. The metal-matrix material may also contain diamond grit in certain areas. Carbide elements may be attached to the body at various points to resist abrasive wear. Thermally stable polycrystalline (TSP) diamond members may also be attached to the body to resist abrasive wear, such as along the gage surface.
Hardfacing has normally not been applied to matrix body diamond bits. The high temperature for the prior art hardfacing process excessively melts the binder of the bit body metal-matrix material. Also, hardfacing has not typically been employed on diamond bit abrasive elements, such as cemented tungsten carbide inserts or tungsten carbide bricks. The high hardfacing temperature melts the binder of these members, which is typically cobalt, and also can cause the members to crack during cool down. In addition, if natural diamonds and/or diamond grit are employed in the metal-matrix of the body, the high temperatures of iron-based hardfacing causes the natural diamonds and synthetic diamonds to revert to carbon and form a carbon dioxide gas. The carbon dioxide gas creates a poor hardfacing layer. The high temperature for iron-based hardfacing has thus precluded its use as a hardfacing for a crown of a diamond bit.
Diamond bits have complex shapes and are very costly. Normally, after the bits are used in drilling, they become worn and require repair in order to be re-used. This repair might involve replacing any damaged or missing polycrystalline diamond cutting elements as well as replacing missing abrasive elements. The repair process can be time consuming and expensive.